Optical applications of thin metal oxide films start from a single layer heat reflecting glass or an anti-reflecting film and further extend to various fields including, for example, an anti-reflecting coating, a reflection-increasing coating, an interference filter and a polarizing film, of a multilayer film type designed so that a light having a specific wavelength is selectively reflected or transparent. The spectral characteristics of a multilayer film are optically designed by using refractive indices n and thicknesses of the respective layers, as parameters and usually adjusted by a combination of a high refractive index film and a low refractive index film. To realize excellent optical characteristics, it is better that the difference in the refractive index n is large between the high refractive index film and the low refractive index film. For example, a combination of titanium dioxide with n=2.4, niobium pentoxide with n=2.3, tantalum pentoxide with n=2.1 or tungsten trioxide with n=2.0, with magnesium fluoride with n=1.38 or silicon oxide with n=1.46, is preferred. Such a single layer or multilayer film can be deposited, for example, by a vacuum evaporation method or a wet coating method.
On the other hand, in a case where a single layer or multilayer film is deposited on a substrate having a large area, such as glass for buildings, glass for automobiles, CRT or a flat display, a sputtering method, particularly, a DC sputtering method, is used in many cases.
In a case where a high refractive index film is deposited by a DC sputtering method, it is presently common to employ so-called reactive sputtering, wherein sputtering is carried out in an atmosphere containing oxygen, but using a metallic target having electrical conductivity. However, the deposition rate for a thin film obtained by this method is extremely slow, and there has been a problem that the productivity is poor, and the cost tends to be high.
In order to avoid such a problem, it is conceivable to put a high sputtering electric power. However, in a case where cooling of the target material cannot catch up, the possibility for a trouble such as cracking or peeling tends to be high, whereby the power which can be put, has been limited. In addition, the stress of the metal oxide film thereby obtained, is high which used to cause bow of the film-coated substrate. Especially when a multilayer film having a metal oxide film and a silicon oxide film alternately laminated in many layers, is formed on a thin substrates the bow tended to increase as the total film thickness increased. Accordingly, such a measure has been taken that bow is prevented by depositing the film on a substrate thicker than the finally required film thickness, then slits of desired sizes are imparted from the film side to release the stress, whereupon the substrate is polished to the necessary thickness, and thereafter, the substrate is cut into a desired size.
Further, a method has also been proposed wherein a transparent metal oxide film having a high refractive index is deposited on a transparent substrate by carrying out DC sputtering by using, as the target material, a metal oxide MOX (wherein M is at least one metal selected from the group consisting of Ti, Nb, Ta, Mo, W, Zr and Hf) which is deficient in oxygen than the stoichiometric composition (e.g. WO (republished) 97/08359). However, this publication discloses nothing about the bow or stress of the film.
It is an object of the present invention to provide a multilayer film-coated substrate having the stress of the film relaxed i.e. having a low stress, by depositing a multilayer film comprising a metal oxide film and a silicon oxide film on a substrate at a high speed by a sputtering method using a conductive sputtering material, and a process for producing a multilayer film-coated substrate having such a low stress. In order to obtain a film-coated substrate with little bow of the substrate, the stress of the multilayer film having a metal oxide film and a silicon oxide film laminated, is required to be from −100 MPa to +100 MPa, preferably from −70 MPa to +70 MPa, particularly preferably from −60 MPa to +60 MPa, most preferably from −30 MPa to +30 MPa. Here, when the stress is on the (+) side, it is a tensile stress, and when it is on the (−) side, it is a compressive stress.